I think people get a bit carried away with the replacement for Fern. He is a supreme talent and has brought us so much joy but if you are expecting a like for like replacement, there doesn't seem to be anyone in world football with his range of attributes so you are going to be underwhelmed by anyone!
He is replaceable, but not by anyone with the exact same skill set. That does not mean that they cannot do as good a job.
An example is my personal belief that we play better as a team with Gabriel Jesus in the team than Sergio. Now Sergio looks almost irreplaceable; he is dynamite on the ball and has consistently topped goalscoring charts. He is a superbly talented player and we are lucky to have him. Jesus is a different player. Are we worse with him in the team? Not for me, we are not. He is not as dynamic and certainly doesn't yet have Sergio's reputation but our results with him in the starting line up are excellent because, in my opinion, he brings more from others than Sergio, who is more of an individual.